Natural areas
Wetlands
Bexley Wetland
Bexley Wetland Trust Newsletter Autumn 2000
Project fresh water pond.
The Trust has been successful in obtaining funds Reserve
from Lottery and the Pub Charity. (see financial statement)
to landscape the East-Bank of the Fresh Water pond near
Wairoa Reserve. John Marsh has designed the landscape plan
and Riccarton Bush, Wai-ora Trust and Trees for Canterbury
supply plants. Our supporters are invited to our community
planting days, scheduled on Saturday 27th May, and the 24th
of June. We will mulch immediately to keep the weeds down.
On Friday the 2nd of June pupils of St James School will
come and plant some areas as part of their contribution
toward Arbour Day.
Dates for your diary Saturday 27th May and 24th
June
Community Planting of the East Bank of the freshwater pond.
Meeting in Wairoa Reserve at the end of Wairoa street at
10.00 AM. Gumboots are not strictly necessary, as it is
on the high slope of the bank but bring your spade.
BBQ will be provided.
The City Council Lottery grant
The Council has applied and received from the Lottery $25,000
for the Bexley Wetland to be used for information signs,
hides etc.
What happened last summer
On Wednesday 2nd of February around 40 supporters met in
the Pleasant Point Yacht Club and discussed their concerns
about the priorities of the restoration and development
of the Bexley Wetland.
Trevor Partridge, Landcare botanist, explained that to
have a maximum of biodiversity we need tidal inundation
and create a sensible zonation pattern to provide the right
environmental conditions for a series of communities.
Andrew Crossland, Ornithologist, explained the importance
of the Bexley Wetland as one of the few remaining high tide
roosting and nesting sites in the estuary, in particular
at spring tides. To provide ample feeding and predator free
nesting opportunities, he suggested to have waterways throughout
the site and create many islands.
Most of the participants of the meeting agreed that it
was important to preserve the open space character as well
as the biodiversity of the freshwater and saline environments.
Trees for Canterbury
Trees for Canterbury donated 800 rushes to the Bexley Wetlands
and their full team of staff members came out on Saturday
morning the 19th February to plant them. Together with staunch
supporters of the Bexley Wetlands all 800 rushes were planted,
even though the soil was much more compacted than was expected.
Project information sign
The Community Trust has granted the Bexley Wetland Trust
$2.400 for an information board to cover the cost of the
material for a traditional wooden information board with
a roof. The Trust invites the carpenters among our supporters
with some spare time on their hands to contact either Hap
Hill Ph 388 8871 or Mia Ph 3822 987
Weeding program
We have put a one-day a week weeding program in place for
a ten week trial. To be successful we need to mulch as soon
as possible after weeding. Supporters who would like to
assist us with mulching, please phone either Hap or Mia
and leave your name and phone number, so we can contact
you for your support when we have set the mulching days.
New members on our Board of Trustees.
We are very pleased to welcome Scott Butcher and Trevor
Partridge to our Board of Trustees.
Trevor Partridge is a plant ecologist working for Landcare
Research at Lincoln. His botanical interests started however
in the salt marshes of Otago, especially at Aramoana, at
the time of the proposals to build aluminium smelters there.
His doctorate thesis was on salt marsh vegetation of Otago,
part of which involved measuring the salt tolerance of virtually
all the marsh plants, a tool that has proven especially
valuable since. After being exiled to Canterbury in 1981,
he has worked on both the Avon/Heathcote estuary, especially
with Kate McCombs of CCC, and the lagoon at Lake Ellesmere.
He also has an interest in other coastal ecosystems, especially
sand dunes. He has recently carried out an assessment of
the restoration plantings in the lower Avon and Heathcote
Rivers for CCC, and this insight has proven especially useful
in planning the restoration of Bexley Wetland.
Scott Butcher is an ornothologist and a member of the N.Z.
Ornithological Society. He has a real interest in birds
in Wetlands. Scott is studying at present in Lincoln for
his B.R.S. He has worked briefly for Landcare Research and
the Department Of Conservation. If you have any questions
or concerns about birds you can phone Scott on 329 5699
or contact him by E-mail: butcher_scott@hotmail.com
Understanding the Bexley Wetland system
By Trevor Partridge
One of the great difficulties regarding the Bexley Wetlands
is knowing exactly what kinds of vegetation are appropriate
for the area, because it lies on that very difficult transition
zone from freshwater to saltwater vegetation. If we go out
to the estuary itself, we see the vegetation known as salt
marsh. Here the rise and fall of the tide dominates the
hydrology , and the plant zonation patterns are mostly in
response to the salinity regime that this water brings.
If we go upstream into the freshwater areas such as Cockayne
Reserve, we have freshwater vegetation in response to hydrological
factors such as water table and groundwater flows. Between
the two, there is a gradual replacement of one with the
other. The difficult task in restoring vegetation is working
out the balance of fresh and salt water and their effects
on vegetation.
This problem would be difficult enough if it were not for
one important complicating factor, that is we are trying
to create vegetation patterns in an area that is currently
out of phase with the appropriate environmental conditions.
This is because there is presently only a narrow opening
to the estuary in a single location beneath the walkway,
and that opening is often closed by a valve. Across where
the walkway is now, there would have once been a continuous
gradation of salt marsh vegetation from saline to brackish
(neither sea water nor fresh, but in between) situations.
The building of the walkway had impounded the Bexley wetland,
creating vegetation that is quite different on either side
of the walkway. On the river side, the vegetation is salt
marsh, and still in contact with the estuarine tides. On
the inland side, the system has become less saline and many
weeds and freshwater species have come in to replace the
estuarine plants. If this situation had continued, the replacement
would have eventually become complete.
However, many of the salt marsh plants are still there;
the taller species such as sea rush and coastal ribbonwood
are hanging on in patches, while many of the herbaceous
salt marsh species can be found on evaporation pans.
An important aspect of the wetland restoration is therefore
to return the appropriate tidal regime to the system behind
the walkway. The building of the "moat" around
the northern margin has already let in a substantial amount
of salt water, provided the valve is kept open. However,
this will be insufficient to get enough salt water to the
rest of the system, as it too is repaired. To do that will
need to involve cutting new and wider openings in the walkway
stopbank to let sufficient waters both in and out of the
wetland. Such openings need to be many and large, an examination
of the outlets at McCormacks Bay and South New Brighton
will show that narrow openings only return a fraction of
the saline waters needed. The difficult part will be to
determine where such openings are to be placed. This is
because the salt marsh on the estuarine side has spread
in response to the walkway and now occupies a continuous
zone of vegetation there. The opening to the present flow
under the walkway is but a narrow channel cut through this
marsh. To return the flows to the Bexley Wetland, it may
be necessary to see a loss of some of that salt marsh vegetation,
especially around the new openings. Considering the excellent
condition of that salt marsh, such a loss will be unfortunate,
and therefore this process of opening the wetland to tidal
influence needs careful consideration and discussion before
it is undertaken.
Once the tidal regime is restored, then the balance of
saline and freshwater inputs can allow restoration to be
better planned. In the northwestern corner, springs of freshwater
will allow species tolerant of such conditions to grade
into the salt marsh further down. The patch of raupo in
the northeastern corner is spring-fed, but is maintained
as a separate, freshwater system by a narrow outflow and
a step in elevation so that salt water does not backflow.
Coupled with the distinctive up slope vegetation patterns
in response to salinity and inundation, this will allow
for considerable diversity in plant and animal species able
to occupy the restored wetland, and should provide a valuable
area to the city.
Trevor Partridge.
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